JP2007-274820A published by the Japan Paten Office proposes a linear actuator which is mounted on a railroad car to produce an damping force. This linear actuator is provided with a cylindrical outer tube having a bottom and a cylindrical inner tube having a bottom that is inserted so as to be free to slide in the outer tube.
The linear actuator disclosed in JP2007-274820A is provided with a columnar inner rod which is disposed on a common axis with the inner tube and fixed to a bottom portion thereof and a cylindrical outer rod which is disposed on a common axis with the outer tube and fixed to a bottom portion thereof. The inner rod is inserted into the outer rod in such a manner that the inner rod can be moved farther into and backward from the outer rod.
The linear actuator is also provided with a magnetic field generator comprising a plurality of annular permanent magnets arranged and retained along an axial direction on an outer peripheral surface of the outer rod and a plurality of coils placed face to face with the magnetic field generator. In this linear actuator, a thrust for relatively driving the inner tube and the outer tube along the axial direction is produced by a magnetic force generated between the magnetic field generator and the coils.
The linear actuator is further provided with a position sensor for detecting the relative positions of the permanent magnets and the coils and a controller for controlling the magnitude and direction of the thrust by regulating currents that flow through the individual coils. The damping force produced by the linear actuator is controlled by the controller.
The coils are retained on an inner periphery of a cylindrical holder which is fixed to an inner periphery of the inner tube. Specifically, there is formed a mating groove in which the holder is fitted in the inner periphery of the inner tube at an open end side thereof. The mating groove is made of a groove portion in which an outer periphery of the holder correctly fits, the groove portion being formed by increasing the inside diameter of the inner tube, and an annular stepped portion formed at a distal end of the groove portion.
An extreme end of the holder is fitted in the groove portion so as to abut against the stepped portion. A flange with an outer periphery having a greater diameter than the inner tube is fixed to an open end of the inner tube. The holder is thereby prevented from falling off from the open end of the inner tube.
When the linear actuator is most extended, the flange comes into contact with a bearing holder provided on an inner periphery of the outer tube at an open end thereof. The bearing holder retains a bearing that slides an outer periphery of the inner tube. When the linear actuator is most contracted, on the other hand, the flange comes into contact with the bottom of the outer tube, whereby the stroke of the inner rod of the linear actuator is restricted.